1. Field of the Invention
The present invention relates to an optical writing device using a plurality of light emission device array units, and relates to an image forming apparatus such as a digital copier, a printer, and a facsimile apparatus.
2. Description of the Related Art
Japanese Patent Laid-open Application No. 10-86438 and Japanese Patent Laid-open Application No. 7-61035 disclose an optical writing device that is provided with a plurality of light emission device array units, each of which has light emission devices arranged thereon along a line, and is arranged one after another in a staggered manner along a line so as to connect all the lines of the light emission devices.
Japanese Patent Laid-open Application No. 10-86438 teaches an LED head set comprised of a plurality of LED heads arranged in an axial direction of a photosensitive body. This configuration makes it possible to expose the whole extent of a photosensitive area extending in the axial direction of the photosensitive body by utilizing each one of the LED heads.
In this type of conventional art, each light emission device array unit is implemented on a print circuit board having a relatively short length such as a length of an A3-size sheet. A plurality of such light emission device array units are arranged along a line so as to form a longer composite line of light emission devices, thereby forming an optical writing device having a relatively long length. Use of such an optical writing device for illuminating the photosensitive body to form a latent image has the following advantages (a) and (b).
(a) It is possible to write a wide image such as the width of an A0 size sheet. When a wide image having the width of an A0 size is to be printed, an optical writing device may be used that has a single light emission device array unit the size of 1 m or so that is longer than the width of A0 size. Such a light emission device array unit having a length of 1 m would be very expensive if it is to be manufactured such that light emission devices are arranged at a 400 dpi (i.e., dot pitches equal to 63.5 micrometer), for example. Such price increase is brought about by difficulties in maintaining sufficient precision over the whole extent of the light emission device array unit in addition to the needs for larger facilities and a drop of the yield rate. The optical writing device comprised of a plurality of units can obviate this problem. (b) If an optical writing device is implemented by using a single light emission device array unit having the length of 1 m, the entire optical writing device needs to be replaced in an event that even one light emission device for one dot fails. In the case of the optical writing device comprised of a plurality of units, only a light emission device array unit that includes the failed light emission device should be replaced, rather than replacing the whole device.
The configuration of the conventional art has the advantages (a) and (b) as described above. In the optical writing device having a plurality of light emission device array units arranged in a staggered manner along the line of light emission devices, however, alignment of dots at the joint point between adjacent units becomes an issue to be addressed, and so does the warping of each light emission device array unit.
A dot pitch at the 400 dpi is 63.5 micrometer. With this dot pitch, an error of the dot pitch should be smaller than 5 micrometer in order to avoid black or white stripes appearing on the reproduced image. Further, a focal length of a light emission device array. unit is generally much shorter than that of a laser writing apparatus, so that the distance from the photosensitive body needs to be accurate within a tolerance range of 0.1 mm or less.
When a plurality of light emission device array units are connected by use of simple support members or the like in a straightforward manner as in the conventional art, the temperature of the light emission device array units will increase due to a change in ambient temperature, heat generation by the light emission device array units themselves, or a temperature increase within the housing of the image forming apparatus caused by heat generated by other components of the apparatus. As a result, each light emission device array unit exhibits thermal expansion.
If the two ends of each light emission device array unit are fixedly mounted on a support member, the unit is warped by the difference of temperature and thermal expansion factors between the unit and the support member, so that there is a risk of losing the accuracy of a focal distance. In order to avoid such warping, each unit may be fixed only at a single point in the direction of a light emission device line, so that portions not fixed to the support member can slide relative to the support member in the direction of the light emission device line. In such a configuration, however, a gap between adjacent light emission device array units will change, resulting in the dot pitch being changed at the joint point. This may cause black or white lines to appear in the reproduced image.
Accordingly, there is a need for a wide-size and inexpensive optical writing device that can avoid the warping of light emission device array units caused by thermal expansion thereof and undesirable lines appearing on reproduced images, and, also, there is a need for an image forming apparatus using such a device.
It is a general object of the present invention to provide an optical writing device and an image forming apparatus that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by an optical writing device and an image forming apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an optical writing device, including an integral structure of a plurality of light emission device array units securely connected together, the units including respective lines of light emission devices, the lines being formed into a longer line of light emission devices on the integral structure, a position fixing member, and a connection mechanism that fixes the integral unit to the position fixing member only at a single connection point such that displacement of the integral unit relative to the position fixing member in a direction of the longer line is prevented only at the single connection point but unrestricted at any other points.
Further, the present invention provides an apparatus for forming an image, including a photosensitive body, an optical writing device which forms a latent image on the photosensitive body by scanning light on the photosensitive body, and a developing unit which converts the latent image into a visible image, wherein the optical writing device includes an integral structure of a plurality of light emission device array units securely connected together, the units including respective lines of light emission devices, the lines being formed into a longer line of light emission devices on the integral structure, a position fixing member, and a connection mechanism that fixes the integral unit to the position fixing member only at a single connection point such that displacement of the integral unit relative to the position fixing member in a direction of the longer line is prevented only at the single connection point but unrestricted at any other points.